High resolution differential spectrometry system

ABSTRACT

A differential spectrometry system detects very narrow-band spectral features, while providing much higher optical transmittance and signal-to-noise ratios than prior optical-filter-based spectrometer systems. A plurality of light detectors (10a, 10b) detect light that falls within respective wide wavebands. The wide wavebands have overlapping and non-overlapping portions, one of which is the desired narrow waveband. The detector outputs are operated upon to produce an output signal (22) which includes substantially only the desired narrow waveband. In the preferred embodiment, the light detectors (10a, 10b) are implemented with a pair of optical detectors (30a, 30b) and respective optical interference filters (24a, 24b). The filters have substantially identical cut-off wavelengths (λ 2 ) and cut-on wavelengths that are shifted by Δλ with respect to each other (λ 1  and (λ 1  +Δλ), respectively). The detector outputs are differenced with an operational amplifier (33), so that detector signals resulting from spectral features common to both detectors (30a, 30b) are canceled. The remaining signal (36) varies according to the amount of light that falls between wavelength boundaries  λ 1  and (λ 1  +Δλ)!. A preferred method of fabricating the optical interference filters (24a, 24b) is also provided.

This is a division of application Ser. No. 08/480,223 filed Jun. 7,1995, now U.S. Pat. No. 5,624,709.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to spectrometry, and more particularly todifferential spectrometry using discrete optical filters.

2. Description of the Related Art

Optical interference filters are used for a wide variety ofapplications, particularly in the field of spectrometry. For example, anarrow band optical filter may be employed in a system that is designedto detect the presence of a particular chemical compound in theatmosphere. The filter is designed to transmit only the light that fallswithin the absorption band of the desired chemical compound, therebymasking other absorption bands found in the atmosphere.

Interference filters are described in H. A. Macleod, Thin-Film OpticalFilters, Second Edition, Macmillan Publishing Co., N.Y. (1986), pages234-313. They typically comprise a quarter-wave (QW) stack that isdeposited on an optically transmissive substrate by vacuum evaporation.A QW stack consists of alternating high and low refractive indexmaterial layers whose optical thicknesses (refractive index timesphysical thickness) are tailored to produce a 90 degree phase shift inthe light that is transmitted through each layer at a design wavelength.The QW stack is an efficient reflector of light over a wavelength range(stopband) 5 that is centered about the design wavelength, asillustrated in FIG. 1a. The width of the stopband 5 is dependent uponthe ratio of the high and low refractive indices of the alternatingmaterial layers.

On either side of the stopband 5 are regions of high transmittance 6with moderate to severe "ripples" in the shape of the transmittancecurve. An edge filter is constructed by using either side of thetransition between the high reflectance stopband 5 and the hightransmission regions 6. A cut-on or long-wavelength pass (LWP) edgefilter is constructed by modifying the basic QW design to minimize theripple on the long wavelength side of the stopband 5, as illustrated inFIG. 1b. Alternatively, a cut-off or short-wavelength pass (SWP) edgefilter is constructed by modifying the QW design to minimize the rippleon the short wavelength side of the stopband 5, as illustrated in FIG.1c. The wavelength at which the edge (or boundary) of either type offilter appears is controlled by controlling the wavelength at which thecenter of the stopband 5 appears.

An extension of the use of QW stacks is to combine a LWP filter at onedesign wavelength with a SWP filter at a longer wavelength to produce abandpass (BP) filter with the transmittance characteristics shown inFIG. 1d. This type of filter transmits light whose wavelength fallsbetween λ₁ and λ₂. The edges 8 are defined by the LWP cut-on and SWPcut-off wavelengths. Light with a wavelength that falls outside of theedges 8 is reflected by the QW stack.

A problem associated with optical interference filters is that the peakoptical transmittance at the design wavelength decreases as the filter'sbandwidth is made narrower. The reduced optical transmittance reducesthe signal-to-noise ratio to a level which may be unacceptable.

SUMMARY OF THE INVENTION

In view of the above problem, the present invention provides adifferential spectrometry system that can detect very narrow-bandspectral features, while providing much higher optical transmittancethan prior optical-filter-based spectrometer systems.

This is accomplished by providing a plurality of light detectors thatare configured to detect light that falls within respective widewavebands. The wide wavebands have overlapping and non-overlappingportions, one of which is the desired narrow waveband. The detectoroutputs are operated upon to produce an output signal which includessubstantially only the desired narrow waveband.

In the preferred embodiment, the light detectors are implemented with apair of optical detectors and respective optical interference filters.The filters have substantially identical cut-off wavelengths (λ₂) andcut-on wavelengths that are shifted by Δλ with respect to each other (λ₁and (λ₁ +Δλ), respectively). The filters are positioned so that eachdetector receives only the light that is transmitted by its respectivefilter. The detector outputs are differenced with an operationalamplifier, so that detector signals resulting from spectral featurescommon to both detectors are canceled. The remaining signal variesaccording to the amount of light that falls between wavelengthboundaries λ₁ and (λ₁ +Δλ)!.

A preferred method of fabricating the optical interference filtersinvolves coating two substrates with long wavelength pass (LWP) opticalcoatings that have a cut-on wavelength of λ₁, annealing one of thesubstrates so that the cut-on wavelength of its LWP coating is shiftedby Δλ, and coating both substrates with short wavelength pass (SWP)optical coatings that have a cut-off wavelength of λ₂.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1d, described above, are graphs illustrating the opticaltransmittance properties of different types of interference filters.

FIG. 2a is a block diagram illustrating the basic principles of theinvention.

FIG. 2b is a graph illustrating the optical sensitivity of a first lightdetector in FIG. 1a.

FIG. 2c is a graph illustrating the optical sensitivity of a secondlight detector in FIG. 1a.

FIG. 3 is a schematic diagram of a preferred embodiment of theinvention.

FIGS. 4a and 4b are cross-sectional views illustrating successive stepsin a preferred method of fabricating the optical filters of FIG. 3.

FIG. 5 is a graph illustrating typical annealing curves for an opticalcoating that can be used to implement the invention.

DETAILED DESCRIPTION OF THE INVENTION

The basic principle of the invention involves providing a plurality oflight detectors that are configured to detect light that falls withinrespective wide wavebands. The wide wavebands have overlapping andnon-overlapping portions, one of which is the desired narrow waveband.The detector outputs are operated upon to produce an output signal whichincludes substantially only the desired narrow waveband. FIGS. 2a-2cillustrate how the invention is implemented with two light detectors. Afirst light detector 10a is configured so that it is sensitive toportions of incoming light 12 whose wavelengths fall between wavelengthboundaries λ₁ and λ₂ !. FIG. 2b illustrates this graphically, by showinga plot 14 of sensitivity vs wavelength for detector 10a. A second lightdetector 10b is configured so that it is sensitive to portions ofincoming light 12 whose wavelengths fall between wavelength boundaries(λ₁ +Δλ) and λ₂ !, as illustrated in the plot 16 of FIG. 2c.

The outputs of detectors 10a and 10b are sent to a processor 18 alongsignal lines 20a and 20b. The processor 18 calculates the differencebetween the two detector outputs, so that detector signals that resultfrom spectral features common to both detectors are canceled. Theprocessor output 22 varies according to the amount of light withwavelengths between boundaries λ₁ and (λ₁ +Δλ)!. Although the Δλsensitivity difference, as shown in FIGS. 2b and 2c, is implemented atthe cut-on wavelength (λ₁), it could be implemented at the cut-offwavelength (λ₂) without departing from the scope of the invention.

FIG. 3 illustrates a preferred embodiment of the invention. Bandpass(BP) optical filters 24a and 24b transmit light that falls betweenwavelength boundaries λ₁ and λ₂ ! and (λ₁ +Δλ) and λ₂ !, respectively.Filters 24a and 24b are preferably optical interference filters.

BP filter 24a transmits light 29a that falls within its wavelengthboundaries λ₁ and λ₂ ! to optical detector 30a, while BP filter 24btransmits light 29b that falls within its wavelength boundaries (λ₁ +Δλ)and λ₂ ! to optical detector 30b. Detectors 30a and 30b may be of anytype that is sensitive over the wavelength range of interest, such as amercury cadmium telluride or indium antimonide detector if thewavelength range of interest lies between approximately 2.5 to 12microns.

The electrical outputs of detectors 30a and 30b are transmitted topre-amplifiers 31a and 31b over signal lines 32a and 32b, respectively,for amplification. The amplified signals are transmitted to adifferential amplifier 33, preferably an operational amplifier (op-amp)through signal lines 34a and 34b, respectively. Detector signals thatresult from spectral features that are common to detectors 30a and 30bare canceled by the differencing operation at the differential amplifier33. Therefore, the amplifier output 35 varies according to the amount ofincident light between wavelength boundaries λ₁ and (λ₁ +Δλ)!.

The bandwidth of the present spectrometry system is controlled byprecisely adjusting the relative cut-on or cut-off wavelength shift Δλbetween the two optical filters 24a and 24b. Because the individualfilters have relatively large bandwidths, the optical transmittance ofthe system is higher than prior systems that utilize a single narrowband optical filter. This results in an improved signal-to-noise ratio.

A preferred method of fabricating optical filters with the requisiterelative shift in their cut-on or cut-off wavelengths is illustrated inFIGS. 4a and 4b. For illustration, the fabrication of an optical filterset with a relative Δλ shift in their cut-on wavelengths will bedescribed. However, the present method may also be used to fabricatefilters with a relative Δλ shift in their cut-off wavelengths.

In FIG, 4a, substantially identical LWP coatings 38a and 38b with acut-on wavelength of λ₁ are fabricated on substrates 40 and 42. The LWPcoatings are preferably QW stacks that are deposited using well knownphysical vapor deposition techniques. The choice of substrates and QWstack materials will depend upon the particular design wavelength. Thesubstrates 40 and 42 should be optically transparent over the wavelengthrange of interest. For example, germanium is a preferred substratematerial for design wavelengths that fall between approximately 2.5 and12 microns.

To insure that each LWP coating has a cut-on wavelength of λ₁, thesubstrates 40 and 42 are preferably coated together in a common coatingrun. If desired, one large substrate may be coated and then cut into thetwo substrates 40 and 42 after the LWP coating has been deposited.

After the LWP coatings are deposited, one of the substrates 40 and itsrespective LWP coating 38a is annealed in an oven (not shown) to shiftthe LWP coating's cut-on wavelength by Δλ. At a given temperature, theΔλ shift in the cut-on wavelength will saturate over time, asillustrated by the family of curves in FIG. 5. FIG. 5 illustrates Δλ vstime at increasing temperature values T₁, T₂ and T₃. The maximum Δλshift 44 that can be achieved depends upon the annealing temperature,the exposure time and the particular material system. It is preferableto operate near the saturation area 46 of the curve so that the Δλ shiftcan be precisely controlled by adjusting the annealing temperature. Thetime required for saturation, and the preferred annealing temperature,will depend on the materials used for the QW stack. As an illustrativeexample, an annealing temperature of 350 degrees Celsius for two hourswill provide a Δλ shift of approximately 40 nm in an LWP coating made ofgermanium and zinc sulfide. The annealing temperature and time aredependent on the materials used in the coating and on the coatingdesign. They are generally derived empirically as were the values usedin the illustrative example above.

After substrate 40 is annealed, both substrates 40 and 42 are coatedwith respective SWP coatings 48a and 48b, as illustrated in FIG. 4b. Thecoatings have a cut-off wavelength of λ₂, and are preferably QW stacksthat are deposited using the same techniques used for the LWP coatings38a and 38b. To insure that each SWP coating has a cut-off wavelength ofλ₂, the substrates 40 and 42 are preferably coated together in a commoncoating run. Although the SWP coatings may be deposited on either sideof the substrates 40 and 42, they are preferably deposited on the sideof the substrate opposite the LWP coatings. The resulting opticalfilters 24a and 24b have the transmittance properties described above inconnection with FIG. 3.

Numerous other variations and alternate embodiments will occur to thoseskilled in the art without departing from the spirit and scope of theinvention. Although a system that uses a pair of detectorsand-respective optical filters was described as the preferredembodiment, the invention may be practiced with more than two lightdetectors. With respect to the preferred embodiment described above, theoptical filters may be designed so that the relative wavelength shiftbetween them borders the cut-off rather then the cut-on wavelength. Inaddition, although annealing is described as a preferred method ofobtaining the desired cut-on or cut-off wavelength shift, other methodsmay be used, such as by precisely controlling the thickness of the LWPor SWP QW stack layers during the growth stage. Such variations andalternate embodiments are contemplated, and can be made withoutdeparting from the spirit and scope of the appended claims.

We claim:
 1. A differential spectrometry system for detecting lightwhose wavelength falls within a narrow waveband, comprising:a pluralityof light detectors configured to detect light that falls withinrespective wavebands that are wider than said narrow waveband, and forgenerating respective outputs that vary with the amount of light withinsaid respective wide wavebands, said wide wavebands includingoverlapping and non-overlapping portions, with one of said portionscomprising said narrow waveband, and a processor for operating upon saiddetector outputs to produce an output signal which includessubstantially only said narrow waveband.
 2. The system of claim 1,wherein said narrow waveband is the narrower of said overlapping andnon-overlapping portions.
 3. A differential spectrometry system fordetecting light whose wavelength falls between boundaries λ₁, and (λ₁+Δλ), comprising:a first light detector configured to detect light whosewavelength falls between a first set of wavelength boundaries λ₁ and λ₂,and to generate a first output that varies with the amount of lightwithin said first wavelength boundaries, a second light detectorconfigured to detect light whose wavelength falls between a second setof wavelength boundaries (λ₁ +Δλ) and λ₂, and to generate a secondoutput that is proportional to the amount of light within said secondwavelength boundaries, and a processor connected to sense the differencebetween said first and second outputs as an indication of the lightbetween λ₁, and (λ₁ +Δλ).
 4. The system of claim 3, wherein λ₁ <λ₂. 5.The system of claim 3, wherein λ₁ >λ₂.
 6. The system of claim 3, whereineach of said first and second light detectors comprise:an opticaldetector, and an optical filter configured and positioned so that itsrespective detector receives light only within its respective wavelengthboundaries.
 7. The system of claim 6, wherein each of said first andsecond optical filters comprise:an optically transmissive substrate, anda quarter-wave stack on said substrate that is designed to reflect lightthat falls outside of the filter's respective set of wavelengthboundaries.
 8. The system of claim 3, wherein said processor comprisesan operational amplifier.
 9. A method of detecting light whosewavelength falls within a narrow waveband, comprising the stepsof:detecting incident light over a plurality of wavebands that are eachsubstantially wider than the narrow waveband, and at least one of whichincludes the narrow waveband, and operating on said wider wavebands tocancel the wavebands outside said narrow wavebands, and yield a residualsignal across said narrow waveband.
 10. The method of claim 9, whereinsaid narrow waveband comprises an overlapping portion of said widerwavebands.
 11. The method of claim 9, wherein said narrow wavebandcomprises a non-overlapping portion of said wider wavebands.